1. Field of the Invention
The present invention relates to mesophase pitch for use in the production of high-performance carbon fibers and other carbon materials.
2. Prior Art
High-performance carbon fibers are commercially produced chiefly from PAN (polyacrylonitrile). However, PAN is expensive and does not show high yield in carbonization. It has recently been found that carbon fibers which have comparable or better characteristics than those prepared from PAN can be produced from inexpensive pitch, and active efforts are being made to commercialize this method.
There are two types of pitch that can be used as a starting material for the manufacture of carbon materials; isotropic pitch and anistropic pitch.
Carbon fibers produced from isotropic pitch are inexpensive but they suffer from the disadvantage of low strength due to poor molecular orientation. Therefore, high-performance carbon products cannot be produced from isotropic pitch. In contrast, carbon fibers produced from anisotropic pitch called "mesophase pitch" have a higher degree of molecular orientation and exhibit improved mechanical properties in terms of strength and modulus of elasticity. Therefore, with a view to producing high-performance carbon fibers, extensive studies are being conducted on the production of mesophase pitch from catalytic cracked petroleum pitch, petroleum tar pitch or coal tar pitch. When fibers are produced by a melt spinning process using mesophase pitch, the developed aromatic planar molecules are brought into alignment with the fiber axis by the shear force exerted as the pitch passes through nozzle holes. This oriented structure is maintained without being disturbed during the stage of subsequent "stabilization" in which the surface of fibers is oxidized by gradual heating under an air current and during the stage of "carbonization" in which the stabilized fibers are heat-treated in an inert gas atmosphere at temperatures not lower than 1,000.degree. C. It has been confirmed by many experiments that this effect contributes to the production of highly oriented, high-performance carbon fibers.
The portion of pitch which has an optically anisotropic phase (this portion is hereinafter referred to as "mesophase") is insoluble in polar solvents such as quinoline and pyridine and it has so far been considered that mesophase is identical to the component which is insoluble in polar solvents. However, later studies have shown that the portion which exhibits anisotropy when observed under a polarizing microscope is not necessarily the same as the insoluble content but that mesophase contains both components which are insoluble and soluble in polar solvents. Therefore, the term "mesophase" as used herein means that portion of a phase which shows optical anisotropy when observed under a polarizing microscope, and the proportion taken by the area of this optically anisotropic phase under observation with a polarizing microscope shall be called "the content of mesophase", or more simply "the mesophase content".
If the mesophase content of pitch is small, the anisotropic phase and isotropic phase in molten pitch will separate to interfere with the spinning operation. Therefore, the mesophase content of pitch is preferably at least 90%, more preferably 100%. However, an increase in the mesophase content generally causes an increase in the softening point and viscosity of the pitch and renders it difficult to perform spinning with consistent results. The high softening point and viscosity necessitate spinning at elevated temperatures but then the pitch is prone to thermal decomposition or condensation, and the resulting gases and infusible high-molecular weight substances make it difficult to continue spinning operations for a prolonged time with consistent results.
Various methods have been proposed for solving these problems with mesophase pitch. U.S. Pat. No. 4,472,265 shows a method in which the mesophase pitch is partially hydrogenated to reduce the degree of stacking of its molecules to an appropriate degree and the resulting "isotropic pitch" is subjected to spinning. Japanese Patent Public Disclosure No. 18421/1983 shows a method characterized by the use of a unique kind of pitch, or "premesophase" pitch which is isotropic during spinning but which turns anisotropic during carbonization. U.S. Pat. No. 4,208,267 shows a method in which isotropic pitch is subjected to solvent extraction, followed by heating the insoluble matter at 230.degree.-400.degree. C. Japanese Patent Public Disclosure No. 136835/1983 shows a method in which isotropic pitch is heat-treated and the resulting mesophase is filtered off, with the remaining pitch being subjected to another heat treatment. U.S. Pat. No. 4,533,461 shows a method in which pitch is heat-treated to adjust the mesophase content to be within the range of 20-80%, followed by precipitation and recovery of the mesophase. U.S. Pat. No. 4,655,902 shows an optically anisotropic carbonaceous pitch that has an average molecular weight of 900-1500, an aromatic carbon fraction (fa) of at least 0.7, a softening point of 230.degree.-320.degree. C. and a content of optically anisotropic phase of 80-100%, and a method of producing such a carbonaceous pitch from specific tar-like materials.
While these methods are improved in one way or another in the use of mesophase pitch, they still have the problems to be described below and completely satisfactory results are yet to be attained by these methods.
In the methods described in U.S. Pat. No. 4,472,265 and Japanese Patent Public Disclosure No. 18421/1983, spinning is performed on isotropic pitch which is not highly oriented, so the molecular orientation in the fibers is not as high as in the fibers spun from anisotropic pitch and the fiber performance is rather low in such aspects as strength and modulus of elasticity. In addition, the method involving the hydrogenation of highly viscous pitch in which the polymerized molecules of a condensed polycyclic aromatic compound are stacked one on another is complicated and is not advantageous for industrial applications.
In the method described in U.S. Pat. No. 4,208,267, only a small amount of the insoluble matter is extracted with a solvent and the yield of mesophase pitch is low. The method described in Japanese Patent Public Disclosure No. 136835/1983 has the disadvantage of complexity in the procedure of filtration to be performed after the heat treatment. The method described in U.S. Pat. No. 4,533,461 involves technical difficulty in recovering the mesophase and suffers the disadvantage of low yield in carbonization.
In the method described in U.S. Pat. No. 4,655,902, a complicated separation procedure is required in order to obtain an optically anisotropic carbonaceous pitch with a softening point as low as 230.degree.-320.degree. C. The pitch thus produced, however, has a fairly broad molecular weight distribution, which means that the pitch has relatively low homogeneity. Namely, it contains molecules having a molecular weight of up to 600 within the range of from 30 to 60 mole %, molecules having a molecular weight of from 600 to 1500 within the range of from 20 to 50 mole % and molecules having a molecular weight of at least 1500 within the range of from about 15 to 35 mole %.
As already mentioned, the mesophase content of mesophase pitch for use in the production of carbon materials has to be increased in order to provide high performance in such aspects as strength and modulus of elasticity. The mesophase content of the pitch must also be increased for the purpose of facilitating spinning operations in the production of carbon fibers. Additional requirements include high heat stability during spinning operations, high stabilization reactivity of the spun fibers and high yield in carbonization. In the case of producing carbon materials, the yield of the carbon material produced by carbonization of the pitch must also be high.
Therefore, the mesophase pitch for use in the production of carbon materials is required to satisfy the following conditions: (1) high mesophase content, (2) high heat stability during spinning operations, (3) high stabilization reactivity, and (4) high yield in carbonization.